The present invention relates to a method of producing a semiconductor device in which a semiconductor chip and bonding wires for taking out electrodes from this semiconductor chip are covered by a silicone gel layer where this silicone gel layer is sealed by a resin from the outside ambience, and, more particularly, to a method of producing a semiconductor module for electric power in which the main circuit of a power converting section is integrated as an individual changeable part.
A conventional producing method of this kind will be described with respect to an example in the case where it is applied to a transistor of the resin sealing type with an insulative radiant plate having a simple structure. FIG. 1 illustrates a cross-sectional view of such a resin sealing type transistor.
(1) Firstly, a structure made of a ceramic substrate 11 is formed. Three independent copper plates 1a, 1b and 1c serving respectively as a collector, an emitter and the base electrodes of the transistor are directly adhered to one surface of this ceramic substrate 11, while a copper plate 1d to which other parts are soldered is directly coupled to the other surface. PA0 (2) Secondly, a transistor chip 13 is mounted by a solder 12 on the copper plate 1a serving as the collector electrode. The other two copper plates 1b and 1c are connected to the respective emitter and base electrodes of the transistor chip 13 by bonding wires 14.sub.1 and 14.sub.2 made of, for example, aluminum (Al). PA0 (3) Then, in the above structure, a base 15 made of copper on which a solder paste having a lower melting point than the solder 12 has been coated, and terminals 16, 17 and 18 for taking out electrodes are set in position so as to satisfy a predetermined arrangement. PA0 (4) Next, each of the above-mentioned components disposed in the predetermined positions is put into a reflow furnace which has been set at a temperature low enough that the solder 12 does not melt. In this way, the terminals 16, 17 and 18 for taking out electrodes and the copper plates 1a, 1b and 1c on one surface on the ceramic substrate 11 are respectively mutually adhered by a solder 19. Also, the copper plate 1d on the other surface on the ceramic substrate 11 and the copper base 15 are mutually adhered by the solder 19. PA0 (5) Then, after the flux containing the solder paste is removed, a cylindrical casing 21 made of resin on the bottom portion of which an adhesive agent 20 has been coated is mounted on the copper base 15, thereby adhering to the copper base 15. This cylindrical resin casing 21 is disposed so as to surround the ceramic substrate 11, the copper plates 1a to 1c attached on the upper portions thereof, and the terminals 16, 17 and 18. PA0 (6) Subsequently, silicone gel from which foam has been almost completely removed is injected from the upper opening of the resin casing 21 into the casing 21 in such a manner that the silicone gel does not come into contact with the terminals 16, 17 and 18, thereby forming a silicone gel layer 22. A sufficient quantity of silicone gel is injected into the casing 21 so as to at least completely embed the bonding wires 14. After the injection, the above-mentioned structure is put in the oven which has been pre-set to 150.degree. C. for one hour or more in order to cure the silicone gel 22. Either a single-liquid or double-liquid layer may be used as the silicone gel layer 22. PA0 (7) Next, epoxy resin, which has been preheated to temperatures of about 70.degree. to 80.degree. C. to seal the silicone gel layer 22 from the outside ambience is injected into the cylindrical casing 21 of the semimanufactured product which was left at an ordinary temperature through its upper opening. Thus, an epoxy resin layer 23 is formed on the silicone gel layer 22. The quantity of the epoxy resin used may be generous, but it must not overflow from the upper opening of the resin casing 21. PA0 (8) Then, in order to completely harden the epoxy resin layer 23, the semimanufactured product in which the epoxy resin layer 23 has been formed is put in an oven pre-set at 150.degree. C. and cured for at least eight hours. As described above, the manufacture of a transistor such as shown in FIG. 1 is completed.
In step (7), the epoxy resin was injected after it was preheated to a temperature of about 70.degree. to 80.degree. C. The reason for this will now be described with reference to FIG. 2. FIG. 2 shows the characteristic curve which represents the relation between the temperature of the double-liquid thermosetting type epoxy resin and its viscosity.
As is obvious from FIG. 2, the viscosity of the epoxy resin remarkably changes depending upon the ambient temperature. At temperatures below temperature b (about 100.degree. C.) where the cure acceleration reaction occurs, the viscosity gradually decreases as the temperature approaches b. The viscosity becomes minimal at temperature b. At temperatures over b, the cure rapidly advances, and the viscosity increases towards infinity. In FIG. 2, T.sub.0 indicates an ordinary temperature. Since epoxy resin shows such a temperature-viscosity characteristic as shown in FIG. 2, it is injected into the product which has been left at an ordinary temperature while the epoxy resin is kept below "b" (for example, a temperature of "a" (about 70.degree. to 80.degree. C.) in FIG. 2). At temperature "b", the resin has a stable viscosity and does not harden for a long time.
On the other hand, in order to check the reliability of the resin sealing type transistor manufactured by the conventional method as described above, we performed temperature cycle tests at -40.degree. C. to +125.degree. C. in a product guarantee temperature range in accordance with the actual installation environment. As a result, the solders 19 for adhering the terminals 16, 17 and 18 and the copper plates 1a, 1b and 1c on the ceramic substrate 11 cracked, and separated from each other. Remarkably, we found that the adhesive between the resin casing 21 and the copper base 15 peeled off, so that they too could have separated. In addition, in the case where the thickness "h" of the epoxy resin layer 23 is small, there were cases such that the silicone gel overflowed from the gaps between the terminals 16, 17 and 18 and the epoxy resin layer 23, and between the resin casing 21 and the epoxy resin layer 23. The inventors of the present application have studied the causes of this phenomenon and have found that it occured due to a difference in the temperature change hysteresis between the epoxy resin layer 23 and the silicone gel layer 22. In other words, it has been confirmed that although a product at an ordinary temperature is put into an oven heated at about 150.degree. C. as in step (8) as described before in order to completely cure the epoxy resin layer 23, there is for a short time a large temperature change hysteresis difference between the epoxy resin layer 23 and the silicone gel layer 22. The difference in temperature causes the above-mentioned cracks and peeling to occur. In general, 3 to 4 minutes after the resin temperature has passed about 100.degree. C. (temperature at point "b" in FIG. 2) where the cure acceleration reaction starts, the epoxy resin 23 is about 80% cured. Thus, the epoxy resin 23 is firmly adhered to the resin casing 21 and to terminals 16, 17 and 18. However at this time, the silicone gel layer 22 reaches a temperature of up to about 70.degree. C. At temperatures below this, the thermal expansion coefficients of the other parts, i.e., the ceramic substrate 11, copper plates 1a, 1b and 1c, terminals 16, 17 and 18, etc. are all less than 1/2 of the silicone gel layer 22, so that the increase in volume due to thermal expansion of those other parts is not so large. Therefore, as described above, if temperature cycle tests at -40.degree. C. to +125.degree. C. in the product guarantee temperature range are performed, a large internal pressure will not be caused in the silicone gel layer 22 at temperatures of less than 70.degree. C. However, an extremely large internal pressure can happen in accordance with the rule of "pressure.times.volume=constant" at temperatures over 70.degree. C. Due to this, tensile stress is applied to the solders 19 by which the terminals 16, 17 and 18 which are weak with respect to structure and the copper plates 1b, 1a and 1c are coupled. This tensile stress is repeatedly applied in the actual installation environment, so that the solders 19 crack and the parts separate due to the peeling of the solders. Furthermore, the adhesive of the resin casing 21 is peeled off, causing the resin casing 21 and the copper base 15 to be separated.
In addition, in a semiconductor device with such a constitution, the exothermic temperature range of the semiconductor chip 13 due to the on-off operation of the transistor is between about 25.degree. C. and 150.degree. C. Due to this, it is apparent that even by this on-off operation, the separation and the like of the parts adhered by the solders 19 or adhesive 20 occurred due to the cracks of the solders 19, the peeling of the adhesive 20, etc.
On the other hand, assuming that the parts and materials which are directly related to the occurrence of cracks in the solders are as shown in the following TABLE I, the elongation amounts of the structure as shown in the following items (A) to (C) due to the thermal expansion in the longitudinal directions of the terminals 16, 17 and 18 were calculated with regard to the temperature difference between 25.degree. C. and 125.degree. C., i.e., the temperature difference of 100.degree. C. Thus, the following results were obtained. That is to say:
(A) solders+terminals for taking out electrodes (the lower surface of the solders 19 for the terminals 16, 17 and 18 are used as references) EQU 23.06.times.10.sup.-3 mm
(B) casing+adhesive EQU 36.45.times.10.sup.-3 mm
(C) silicone gel+epoxy resin EQU 62.00.times.10.sup.-3 mm
The differences between the elongation amounts due to the thermal expansion which causes the occurrence of cracks in the solders are: EQU (B)-(A)=13.39.times.10.sup.-3 mm EQU (C)-(A)=38.94.times.10.sup.-3 mm
Therefore, even when taking into consideration the amount of mechanical elongation of 23% of solder, if the parts mutually coupled by the solder exhibit an amount of elongation due to thermal expansion which is greater than (A)+0.1.times.0.23.apprxeq.46.times.10.sup.-3 mm, the solder will crack. The above calculation results represent the elongation amounts for the entire lengths of the structures of (A), (B) and (C), respectively.
Next, the thermal expansion elongation amounts of the structures shown in the following items (A)', (B)' and (C)' (each corresponding to only the portion which is located in the silicone gel layer 22 having a thickness of 5 mm) were calculated; however in this case, the regions fixedly adhered by the sealing epoxy resin layer 23 were ignored. Thus, the following results were obtained at the temperature difference of 100.degree. C.:
(A)' solders+terminals for taking out electrodes EQU 11.94.times.10.sup.-3 mm
(B)' casing+adhesive EQU 18.3.times.10.sup.-3 mm
(C)' silicone gel EQU 37.5.times.10.sup.-3 mm
The difference (C)'-(A)' is EQU (C)'-(A)'=25.56.times.10.sup.-3 mm;
It will be appreciated that this difference causes the solders to crack even if the mechanical elongation of 23% is considered.
TABLE I ______________________________________ Coefficient Length According Parts & of Thermal to Conventional Materials Material Expansion Example (mm) ______________________________________ Silicone gel Silicone 75 .times. 10.sup.-6 5 layer 22 Terminals for Copper 19 .times. 10.sup.-6 12 taking out electrodes 16, 17, 18 Epoxy resin Epoxy 35 .times. 10.sup.-6 7 layer 23 Resin casing PBT 30 .times. 10.sup.-6 12 21 Solders 19 37 pb 26.3 .times. 10.sup.-6 0.1 -63 Sn Adhesive Silicone 45 .times. 10.sup.-6 0.1 rubber ______________________________________